This invention relates to a propfan turbine engine for aircraft and, more particularly, to a propfan turbine engine having a shroud which surrounds the propfan blades. The shroud includes a leading edge which, when viewed with respect to the aircraft, has an upper, annular section designed as a separate profile part and being variable with respect to the shroud via an actuating device.
With known propfan turbine engines, the approximate axial Mach number between the fan entry and exit ranges between 0.75 and 0.78, whereas with conventional turbofan engines the approximate axial Mach number is typically 0.65 at the entry and 0.45 at the exit. With the conventional turbofan the more pronounced deceleration of the oncoming in-flight air flow in its passage through the fan necessitates a substantially more pronounced curvature of the outer contour of the fan shroud, or a larger maximum diameter of the fan shroud relative to the diameter of the oncoming air stream, than with the shrouded propfan.
In the same manner that the slim, aerodynamically clean contour, which can be achieved for at least the counterrotating shrouded propfan, is desirable in normal flight for its low shroud drag, the shrouded propfan is much more sensitive than the shroud contour associated with the conventional turbofan to flow separation at off-design incidence. Still, every effort must be made to prevent flow separations from the shroud inside or outside and at any incidence encountered in flight operation. This holds true at all flight Mach numbers and aircraft angles of attack, and over the entire effective operating range of the engine.
It is especially during takeoff, i.e. during roll and liftoff (rotation), and during climb that incidences in the leading edge area of the shroud may be unfavorable enough to cause flow separations in that area. These separations may occur on both the inside and the outside of the shroud, and also in certain areas, i.e. top or bottom, only or along the entire circumference.
Air flow separations are especially critical and should positively be prevented when occurring in the top position on the outside of the shroud. This is on account of the adverse effect separations have on the approach flow to the wing, which is normally arranged behind that area. This adverse effect occurs at the wide engine or aircraft angles of attack encountered during rotation and climb. Jeopardy attaches also when separation occurs at the bottom position on the inside of the shroud, which is equally critical with respect to the approach flow to the fan.
Solutions which provide variable geometry in the shroud inlet area have been disclosed, e.g. in U.S. Pat. No. 3,446,223, where circumferentially spaced flaps are provided behind the lip to inject air from the outside in an inward/rearward direction. This eliminates separation all around on the shroud inner side so that improvement is only achieved in the rather uncritical flight situation where separation is threatened all around on the inside. In this situation, the fan is inherently capable in a large measure to stabilize the flow at the fan entry.
In another disclosure, the entry portion is designed as an axially movable ring to form an annulus permitting air to flow from the outside in an inward, rearward direction. This second solution accordingly falls in much the same category as that disclosed in U.S. Pat. No. 3,446,223. A design similar to the above proposal has been disclosed in German Patent Specification 20 48 588, where additional consideration must be paid to vibration problems affecting the entry portion, which automatically responds to changes to static pressure. This design also exhibits considerable complexity of mechanical design. Further, it will not permit control of the flow on the upper outer side of the shroud.
There is therefore needed a generic propfan turbine engine design which eliminates the hazard of flow separations in the presence of critical approach flow to the shroud.
This need is met by the present invention which includes a propfan turbine engine for aircraft having a shroud surrounding propfan blades including a leading edge of the shroud. The leading edge includes at least one separate profile part in an upper annular section. An actuating device is coupled to the at least one separate profile part to vary the separate profile part with respect to the shroud. A slot-like duct is formed between the upper annular section and the shroud when the separate profile part is in an extended position. The duct extends along an upward and rearward path.
An advantage of the present invention is provided in that air flow separations can safely be prevented by extending the profile part, or parts if one each is provided at the top and the bottom. As a result, an aircraft fitted with a propfan engine arranged in accordance with the present invention will tolerate wider angles of attack without risking flow separation on the nacelles, which in turn makes for improved flight safety under these critical conditions.
Another advantage is provided in that a generally axisymmetric outer shroud contour can be achieved and that an optimally low-drag, slim shroud contour is realized without necessarily allowing for the above-described off-design incidences. Another consideration is that the contour and geometry of the profile parts can be tailored to the following, especially critical cases of off-design incidence.
The duct created on the top side of the shroud when the profile part is extended is shaped such that the flow enters on the inside of the shroud, passes through the duct in a diagonally aft direction and sweeps the outside of the shroud. This effectively prevents flow separation from the top outer side.
The other duct formed on the lower side of the shroud, when the profile part is extended, is shaped such that the flow can enter from the outside and pass through the duct to eliminate the risk of separation from the bottom inner side of the shroud. The aft contours of the extendible profile parts accordingly are formed differently.
In a further aspect of the present invention the profile parts can pivot about a horizontal axis. For this purpose, one profile part is hinged at its circumferential ends to the shroud, with the actuating device acting on the center of one profile part.
This design provides an advantage in that it is much easier to mechanically manipulate that prior-art flaps or rings. Another advantage is provided in that the duct takes an approximately sinusoidal (i.e. half a sine wave) section corresponding to the desirable magnitude of pulse along the circumference to excite the flow. In this manner, therefore, an air stream distributed over the circumferential section of the profile part is generated, the flow rate of which corresponds to the local tendency for flow separation prevailing in the circumferential area affected.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.